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CO
CC
CN
CN
CC
4000
N—H
O—H
3200
2800
2300
2100
1800
1500
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C-H
Triples
Singles
X-H Attached to heteroatoms
Doubles
cm -1
2380
1460, 1380
4000
3000
CO 2
2000
Nujol
1000
FIGURE 3.2
IR stretching frequency ranges.
are separated from each other and the mass spectrum records the quantity of ions
of particular mass-to-charge ratios. Peak heights recorded on a mass spectrum are
proportional to the number of ions of each mass. In this manner, the readout using
mass spectroscopy is used to determine the molecular weight and help support the
identity and features of the molecular structure.
As an example, the mass spectrum of a simple alkane, pentane, C 5 H 12 is presented
in Figure 3.3. The highest mass peak, m/z 72 mu corresponds to the molecular mass
of the pentane molecule. Subsequent fragmentation yields smaller fragment ions,
which are identified in Table 3.1.
10
20
30
40
m/z
50
60
70
FIGURE 3.3
Mass spectrum of pentane, showing fragment ions.
TABLE 3.1
Mass Spectrum of Pentane
m/ z
Fragment
Loss of
72
CH 3 CH 2 CH 2 CH 2 CH 3
57
. CH 2 CH 2 CH 2 CH 3
CH 3 (m-15)
43
. CH 2 CH 2 CH 3
CH 2 (m-14)
29
. CH 2 CH 3
CH 2 (m-14)
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